The present invention relates to a method for removing nitrogen oxides contained in exhaust gases, particularly an optionally fluctuating composition, with a temperature from 0.degree.-600.degree. C. by treating the exhaust gases with a reducing agent on a catalyst at a preselected stoichiometric ratio between the concentration of reducing agent and the initial concentration of nitrogen oxide.
Nitrogen oxides, which are produced primarily in combustion processes, are among the main causes of acid rain and of photosmog and the damage to the environment associated therewith. These oxides should therefore be eliminated to as large an extent as possible by being removed from exhaust combustion gases before they are released into the environment.
The primary sources of nitrogen oxide emissions are motor vehicle traffic, stationary internal combustion engines, powerplants, heating plants, steam generators for industrial purposes and industrial production plants.
The elimination or decrease of nitrogen oxides takes place as a rule by means of catalytic reduction reactions. Ammonia is the main reducing agent used since it reacts in a largely selective manner with the nitrogen oxides even in an oxygen containing atmosphere.
However, reducing agents which contain or split off ammonia such as e.g. urea and also H.sub.2, CO and hydrocarbons can also be used.
It was found that the conventional catalysts exhibit a more or less distinct adsorption or storage capability for the reducing agent used in the process, depending on the composition of the catalyst as a function of the processing temperature. This quality can adversely affect the reducing action of the catalyst, e.g. in the case of fluctuating nitrogen oxide contents in the flue or exhaust gas. Diminished nitrogen oxide reduction rates or undesired breakdowns of reducing agent can occur. It was futhermore found that the adsorbed reducing agent blocks active centers of the catalyst and thus decreases the reducing capacity of the catalyst.